1. Field of the Invention
The present invention is directed to a method for the selection of a local antenna from among a number of local antennas that are spatially distributed in a diagnostic magnetic resonance apparatus, for subsequent use for obtaining image data with the apparatus.
2. Description of the Prior Art
For improving the signal-to-noise ratio, and thus the imaging quality, local antennas that are positioned close to the signal-generating slice are utilized in diagnostic magnetic resonance apparatus. Such local antennas, however, have a pronounced directional characteristic and a non-uniform sensitivity distribution. In order to uniformly image an examination region, a number of local antennas (typically, 2-4) are simultaneously utilized. Respective, separate tomograms, that are subsequently combined, are calculated from the simultaneously received magnetic resonance signals of an excited slice. To that end, multi-channel reception paths, each with digitalization, demodulation, data acquisition and calculation must be provided in the magnetic resonance apparatus for the respective antennas.
The question of which of a number of local antennas should be used for imaging of a specific region also arises for a user. To this end, for example, measurement protocols that also prescribe a coil selection can be offered by the controller of the magnetic resonance apparatus for various body regions. These protocols, however, do not adapt to the differing anatomy of various patients. The slice to be measured, which has been set by the user, is not taken into consideration either. Frequently, the predetermined coil selection therefore must be adapted to the current situation, by settings made via a user interface. A non-optimum selection can thereby occur, so that measurements must be repeated because of the poor image quality.
An object of the present invention is to provide a method for the selection of one or more local antennas for a subsequent imaging from among a number of local antennas that are spatially distributed in a diagnostic magnetic resonance apparatus.
This object is achieved in accordance with the invention in a method which includes steps: A target volume for the imaging is excited to emit magnetic resonance signals. The magnetic resonance signals with multiple local antennas. A characteristic quantity in the magnetic resonance signals respectively received by the local antennas is determined, this characteristic quantity representing a measure for the distance of the target region from the respective local antennas. The local antenna is selected, from among the multiple antennas, that is located closest to the target region dependent on the characteristic quantity.
The method for selection is implemented before the actual imaging. A target volume is thereby excited to emit magnetic resonance signals, this target volume being exactly identified by the slice or slices selected by the user. Fundamentally, it has the shape of a cuboid that is oriented arbitrarily in space. This cuboid is defined such from the slices specified for the imaging measurement by the user so that is represents the smallest envelope for the specified slices.
In the general case, the magnetic resonance apparatus has fewer parallel radio-frequency reception channels it than there are local antennas. In an embodiment, following a connection of different local antennas to the respective radio-frequency reception channels, the excitation, reception and the determination of the characteristic quantity are repeated until the received magnetic resonance signals and thus the characteristic quantities, for all local antennas are obtained.
Since no resolution in the readout, phase coding and slice selection directions is needed for the determination of the characteristic quantity of the magnetic resonance signal received from the excited volume, the target volume is excited to emit stimulated echos in an embodiment. The volume-selective excitation thereby ensues with three radio-frequency pulses and orthogonal gradient pulses.
For example, the noise in the signal can be defined as the distance-dependent characteristic quantity. In a further embodiment, however, the amplitude of the received magnetic resonance signal is employed as the distance-dependent characteristic quantity. If the individual local antennas have different sensitivities, the coil selection can be determined from the signal amplitude normalized to the maximum sensitivity.